Fixed #11319 - Added lookup support for ForeignKey.to_field. Also reverted no-longer-needed model formsets workaround for lack of such support from r10756. Thanks Russell and Alex for review.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15303 bcc190cf-cafb-0310-a4f2-bffc1f526a37